NEW YORK (AP) -- The Rev. Al Sharpton opened his annual civil rights conference, which will include a speech by President Barack Obama, amid revelations about his cooperation with the FBI in a 1980s mob investigation.

Sharpton admitted on Tuesday that he helped the feds investigate New York Mafia figures, even making secret recordings that appeared to help bring down a mob boss. The cooperation, revealed Monday in a story on The Smoking Gun website, came at an awkward time for Sharpton, who presided over the National Action Network conference Wednesday.

But New York City Mayor Bill de Blasio, a longtime ally of Sharpton, opened the conference with a full-throated defense of the civil rights leader.

"I just want everyone to know I am proud to stand with Rev. Sharpton," de Blasio told a crowd gathered in a ballroom of a Manhattan hotel. "Because to borrow a phrase from our youth, Reverend, 'you're the real thing.'"

De Blasio heaped praise about Sharpton, the activist-turned political-commentator, and credited him with leading a march two years ago that turned the tide against the NYPD tactic known as stop-and-frisk, which allows police to question someone deemed suspicious.

De Blasio, a Democrat who took office in January, has vowed to reform the tactic and dropped the city's appeal of a court order to have a federal monitor oversee the NYPD.

"(Sharpton's) work gets more powerful with every passing year: He reaches more people, he has a greater impact," the mayor said.

De Blasio's cheerleading was a sign of Sharpton's increased stature in New York's political world. Though Sharpton was at times cordial with de Blasio's predecessor, Michael Bloomberg, he often fought with both the billionaire businessman and ex-Mayor Rudolph Giuliani. Now, he frequently chats with de Blasio, and his former chief of staff now works for the city's first lady.

Sharpton has also had several White House meetings with Obama, who has appeared at the conference in previous years and will deliver its keynote address on Friday.

Sharpton said Tuesday that he went to federal authorities after low-level mobsters warned him and others they would be harmed if they continued to compete for a stake in the music business -- a claim he recounted in his 1996 book, "Go and Tell Pharaoh."

But he denied knowing that he was a confidential informant and said he never knew he had been labeled "CI-7" by the FBI.

Attorney General Eric Holder is appearing at the conference on Wednesday. The New York office of the FBI has not commented on the report.